1. Field of Invention
The present invention relates to systems for controlling the thickness of plastic film extruded from a die and particularly to an improved system which controls the heating of die adjusting bolts with greater speed and precision.
2. Description of the Prior Art
Devices for controlling the thickness of plastic film extruded from dies have generally employed adjustable die bolts which control the width of the orifice of the die. Mechanical means for adjusting the die bolts are shown in U.S. Pat. No. 3,293,689, assigned to the same assignee as the instant application, while electrical means for utilzing the thermal expansion and contraction of the adjustable bolts are described in U.S. Pat. No. 2,938,231. Heating of the bolt is done by an electrical heater on each bolt and is controlled manually or automatically by a gauging device. Increasing the electrical current raises the temperature of the bolt causing it to lengthen and reduce the width of the die opening, while decreasing the current allows the bolt to cool and contract to increase the opening.
With the introduction of automatic gauging devices, it became advantageous to utilize the signals from the gauge to automatically adjust the die orifice to maintain uniform firm thickness. One system for accomplishing this is found in U.S. Pat. No. 3,122,784 wherein signals from a gauging device actuate adjuster motors on each of the die adjuster bolts. This approach, however, was cumbersome and costly since it required a great many closely spaced high torque motors.
Another automatic adjustment system is disclosed in U.S. Pat. No. 3,940,221 which utilizes electrical means to control the voltage and current to the die adjusting bolt heaters and cooling means to cool the bolts and decrease the reaction time. Changes in film thickness are sensed automatically and current and voltage to the heaters increased or decreased as required until the film thickness returns to the set thickness. The length of the die adjusting bolt is thus determined by the amount of heat in the form of current supplied to it. The latter patent also shows the system employed for producing tubular extrusions as well as flat sheet and web forms.
A further system for controlling temperature of plastic extruders in shown in U.S. Pat. No. 4,272,466 in which a sensed temperature is compared with a manually preset temperature. However, this is not used for control of film thickness or the length of die adjusting bolts.
These prior art systems have disadvantages in that the reaction time for changing the length of the die adjusting bolt is unacceptably long for commercial operation. The time for cooling and reducing the bolt length is particularly long and where a continuous source of external cooling fluid is provided, this is expensive and inefficient since heating and cooling means are used at the same time. In addition, changes in external conditions such as ambient temperature, drafts, and line voltage variations influence the temperature of the die adjusting bolts.